Turbochargers are well known and widely used with internal combustion engines. Turbochargers convert energy of the engine exhaust gas to an increased supply of charge air to the cylinders of the engine. Generally, turbochargers supply more charge air for the combustion process than can otherwise be induced through natural aspiration. The increased charge air supply allows more fuel to be burned, thereby increasing power and torque obtainable from an engine having a given displacement and improved performance characteristics from available production engines. Additional benefits include the possibility of using lower-displacement, lighter engines with corresponding lower total vehicle weight to reduce fuel consumption. Some turbocharger applications include the incorporation of an intercooler for removing heat (both an ambient heat component and heat generated during charge air compression) from the charge air before it enters the engine, thereby providing an even more dense air charge to be delivered to the engine cylinders. Intercooled turbocharging applied to diesel engines has been known to at least double the power output of a given engine size, in comparison with naturally aspirated diesel engines of the same engine displacement.
Additional advantages of turbocharging include improvements in thermal efficiency through the use of some energy of the exhaust gas stream that would otherwise be lost to the environment, and the maintenance of sea level power ratings up to high altitudes.
At medium to high engine speeds, there is an abundance of energy in the engine exhaust gas stream and, over this operating speed range, the turbocharger is capable of supplying the engine cylinders with all the air needed for efficient combustion and increased power and torque output for a given engine construction. In certain applications, however, an exhaust gas waste gate is needed to bleed off excess energy in the engine exhaust gas stream before it enters the turbocharger turbine to prevent the engine from being overcharged with air. Typically, such waste gates are set to open and to limit exhaust gas energy at a pressure below which undesirable predetonation or an unacceptably high internal engine cylinder pressure may be generated.
At low engine speeds, such as idle speed, however, there is disproportionately little energy in the exhaust system than may be found at higher engine speeds, and this energy deficiency prevents the turbocharger from providing a significant level of charge air boost in the engine intake air system. As a result, when the throttle is opened for the purpose of accelerating the engine from low speeds, such as idle speed, there is a measurable time lag and corresponding engine performance delay, before the exhaust gas energy level rises sufficiently to accelerate the turbocharger rotor and provide the compression of intake air needed for improved engine performance. The performance effect of this time lag may be pronounced in smaller output engines which have a relatively small amount of power and torque available before the increased turbocharger output provides the desired compression. Various efforts have been made to address this issue of time lag, including reductions of inertia of turbocharger rotors.
In spite of evolutionary design changes for minimizing the inertia of the turbocharger rotor, however, the time lag period is still present to a significant degree, especially in turbochargers for use with highly rated engines intended for powering a variety of on-highway and off-highway equipment.
Furthermore, to reduce exhaust smoke and emissions during acceleration periods, when an optimal fuel burn is more difficult to achieve and maintain as compared with steady-speed operation, commercial engines employ devices in the fuel system to limit the fuel delivered to the engine cylinders until a sufficiently high boost level can be provided by the turbocharger. These devices can reduce excessive smoking, but the limited fuel delivery rate is a further cause of sluggishness in engine performance.
The turbo-lag period can be mitigated and, in many instances, virtually eliminated by using an external power source to assist the turbocharger in responding to engine speed and load increases. One such method is to use an external electrical energy supply, such as energy stored in d.c. batteries, to power an electric motor attached to the turbocharger rotating assembly. The electric motor can be external and attached to the turbocharger rotor through a clutching mechanism, or it can be added onto the turbocharger rotating assembly and energized and de-energized through the use of appropriate electronic controls.
Patents disclosing turbocharger-electrical machine combinations include U.S. Pat. Nos. 5,406,797; 5,038,566; 4,958,708; 4,958,497; 4,901,530; 4,894,991; 4,882,905; 4,878,317 and 4,850,193.
In some turbocharger-electrical machine combinations, permanent magnets, as electrical machine rotor elements, have been attached to the turbocharger shaft. The attachment of the permanent magnets to the turbocharger shaft has a major disadvantage in that the magnets are subjected to heat which is conducted along the shaft from the hot turbine wheel of the turbocharger, and the permeability of the magnets may be reduced by such heating to a level which may be unacceptable for efficient operation of the electric machine. When the turbocharged engine is subjected to a hot shutdown and the lubricating oil flow through the turbocharger bearings and over the turbocharger shaft is interrupted, a steep temperature gradient will exist for a significant length of time until the hot parts of the turbocharger are drained of their heat content.
In other turbocharger-electrical machine combinations, permanent magnet machine rotor elements have been mounted on the aluminum compressor wheel of a turbocharger outboard of the turbocharger shaft bearing system. The addition of motor components such as rotor magnets to the turbocharger compressor wheel, however, results in increasing the overhang of the compressor wheel to such an extent that the stability of the turbocharger bearing system becomes questionable. Most commercial turbochargers in general use on internal combustion engines employ a bearing system in which two floating bushings are used with an outboard stationary thrust bearing.